Traditional middle-of-the-line (MOL) fabrication uses a stack having titanium (Ti)/titanium nitride (TiN), a tungsten nucleation layer, and a tungsten fill layer. TiN is used as a fluorine barrier layer. The tungsten nucleation layer is formed atop the TiN layer by, for example chemical vapor deposition using diborane (B2H6) and tungsten hexafluoride (WF6). Both the TiN barrier layer and the tungsten nucleation layer are highly resistive compared to the tungsten fill layer. The minimum TiN barrier layer thickness is at least about 20 angstroms. The tungsten nucleation layer typically has a thickness of about 10 to about 30 angstroms. As the geometries of electronic devices continue to shrink and the density of devices continue to increase, overall feature size has decreased and aspect ratio has increased. The inventors have observed that as feature sizes shrink to about 10 nm or less, the TiN barrier layer and tungsten nucleation layer occupy a significant amount the via space, leaving little room for low resistive tungsten growth. Furthermore, non-conformal deposition of the TiN barrier layer and tungsten nucleation layer narrows via openings, causing significant challenge for subsequent tungsten fill steps. Therefore, the inventors believe that MOL fabrication requires new processes and integration schemes to meet its resistivity and fill requirements.
Thus, the inventors have provided improved methods for forming metal organic tungsten for MOL applications.